1. Field of the Invention
The present invention relates in general to a hybrid drive system for a motor vehicle, which has shift control means for controlling an automatic transmission to change its speed ratio, according to a predetermined normal shift pattern determined to drive the motor vehicle with an optimum drive force, in a normal operating state of the vehicle, and according to a predetermined special shift pattern in a predetermined special operating state of the vehicle such that the speed ration of the automatic transmission controlled according to the special shift pattern is smaller than the speed ratio controlled according to the normal shift pattern. More particularly, the present invention is concerned with techniques for assuring a sufficient vehicle drive force in the special operating state in which the automatic transmission is controlled according to the special shift pattern.
2. Discussion of the Related Art
An example of such a hybrid vehicle drive system is disclosed in JP-A-7-67208, which includes as a drive power source an engine operated by combustion of a fuel and an electric motor operated with an electric energy a drive power, and an automatic transmission disposed between the drive power source and a drive wheel of the vehicle.
A hybrid vehicle drive system has different operating modes which are selectively established so as to minimize the fuel consumption and exhaust gas emissions. For instance, the operating modes include: an engine drive mode in which only the engine is used as the drive power source; a motor drive mode in which only the electric motor is used as the drive power source; an engine-motor drive mode in which both the engine and the electric motor are used as the drive power source; and an engine drive and charging mode in which the motor vehicle is driven by the engine while the electric motor is operated by the engine as an electric generator or dynamo, for charging an electric energy storage device which is provided to store an electric energy used by the electric motor to drive the motor vehicle.
The automatic transmission provided in such a hybrid vehicle drive system may be an automatic transmission having a plurality of operating positions having respective different speed ratios (input speed/output speed), which are selectively established by selective engagement and disengagement of clutches and brakes. Alternatively, the automatic transmission may be a continuously variable transmission whose speed ratio is continuously variable. The hybrid drive system has shift control means for controlling the automatic transmission to change its speed ratio, so as to drive the vehicle with an optimum drive force, according to a predetermined normal shift pattern on the basis of suitable parameters such as an operating amount of an accelerator pedal and a running speed of the vehicle. This normal shift pattern is used in the normal operating state of the vehicle, without a failure associated with the shifting of the automatic transmission.
The automatic transmission may fail to establish a given operating position due to a failure or defect of a solenoid-operated shift control valve or any other component. To cope with this failure, it is proposed to arrange the shift control means so that the automatic transmission is controlled according to a predetermined special shift pattern in a special operating state of the vehicle, namely, when a given operating position of the automatic transmission cannot be correctly established. According to the special shift pattern, the speed ratio of the automatic transmission is controlled to be smaller than that determined according to the normal shift pattern. An example of this type of shift control means is disclosed in JP-B-61-58694.
Where the second-speed position of the automatic transmission cannot be established, for example, the automatic transmission cannot be shifted up from the first-speed position to the second-speed position, and cannot be shifted down from the third-speed position to the second-speed position. In this case, the automatic transmission controlled according to the special shift pattern is shifted up from the first-speed position to the third-speed position whose speed ratio is smaller than that of the second-speed position, even when the automatic transmission should be shifted to the second-speed position according to the normal shift pattern. Alternatively, the automatic transmission controlled according to the special shift pattern remains in the third-speed position, even when the automatic transmission should be shifted down to the second-speed position according to the normal shift pattern. Since the second-speed position cannot be established when the automatic transmission should be shifted up or down to the second-speed position, the drive force for driving the motor vehicle tends to be insufficient when the automatic transmission is controlled according to the special shift pattern in the special operating state of the vehicle (i.e., upon some failure of the transmission). Namely, the transmission is undesirably shifted up from the first-speed position directly to the third-speed position even when the running condition of the vehicle (operation amount of the accelerator pedal and the vehicle running speed) requires the transmission to be shifted to the second-speed position, or alternatively, undesirably remains in the third-speed position even when the running condition of the vehicle requires the transmission to be shifted down from the third-speed position to the second-speed position. Thus, the vehicle drive force is likely to be insufficient according to the special shift pattern.
The above problem is also encountered where the automatic transmission is a continuously variable transmission.